The invention refers to a method for monitoring the light emitted from an illumination apparatus, and to an apparatus for carrying out said method.
Methods and apparatuses of this kind are used wherever, for reasons of accuracy, the values of the light emitted from the illumination apparatusxe2x80x94for example brightness, brightness fluctuations, spectral properties, and the likexe2x80x94must be kept within narrow parameter ranges. This is the case in particular with optical measuring instruments such as those, for example, for layer thickness determination, in which changes in the measurement light caused by a measured specimen are used to draw conclusions as to the properties and/or dimensional consistency of the measured specimen.
Excellent reliability is important in instruments that are used for dimensional consistency inspection in continuous production lines, for example in the manufacture of wafers in semiconductor production, since the measurement results serve as the basis for obtaining information as to product quality and the stability of the production process. This requires stable accuracy in the measuring instrument technology used.
In instruments that operate on optical principles, measurement accuracy always depends to a considerable degree on consistent parameters of the measurement light that is generated in an illumination apparatus. In the lamps usually used for the purpose, however, the properties of the emitted light change with increasing operating life, so that these lamps become unsuitable for measurement purposes because of their age. For economic reasons, however, it is desirable to use the lamps as long as possible without allowing measurement inaccuracy. For safety reasons as well, it is often not desirable to continue using lamps after a maximum permitted operating life has expired.
All that is known in this regard from the existing art is to sense the failure of a lamp and then to perform a lamp replacement. This is described, for example, in U.S. Pat. No. 3,562,580 A, which refers to a projection apparatus.
From U.S. Pat. No. 4,831,564 A it is also known to estimate the remaining lifetime of a xenon lamp on the basis of the present discharge current. What is utilized here is a predefined relationship between discharge current and lifetime, so that on the basis of the instantaneously sensed discharge current, a theoretical remaining operating life can be determined. Since this method allows absolutely no monitoring of the quality of the light emitted by the lamp, it is unsuitable for use in an illumination apparatus for generating measurement light within narrow quality limits.
U.S. Pat. No. 5,495,329 A furthermore refers to an illumination apparatus for a scanner which, upon startup of the scanner, examines the light emitted by a lamp for the presence of various properties, a high degree of consistency in the luminance over a region being scanned being of paramount importance. In addition, based on the time required for the lamp to warm up, information is obtained concerning the aging status thereof, from which predictions can then be obtained regarding the remaining useful life. In this case as well, however, it is impossible to derive reliable information about the quality of the measurement light or an ideal time at which to exchange the lamps.
It is one object of the invention to create an economical and effective method of making available a measurement light whose properties remain consistent over long periods of time.
This object is achieved by a method which comprises the following steps:
switching on and off multiple lamps of the illumination apparatus wherein the switching is carried out individually for each lamp or in groups of lamps;
sensing of lamp parameters and/or measurement light parameters;
comparing the sensed parameters with predefined setpoints referred thereto;
signaling a deviation in one or more of the sensed parameters from the predefined setpoints beyond a specific tolerance; and
exchanging the lamp or lamp group thereupon.
A further object of the invention is to provide an apparatus for an optical measuring instrument, in particular a layer thickness measuring instrument wherein the apparatus provides contant illumination properties for a long period of time. Moreover, the downtime of the a layer thickness measuring instrument should be reduced.
The above object is achieved by an apparatus which comprises:
multiple lamps defining a measurement light source, of which at least one is provided for performing a measurement task while the others serve as reserve lamps;
an operating voltage source that can be switched on and off and is connected via contacts to the at least one lamp defining the measurement light source;
an activatable device for selectably conveying at least one lamp to the contacts;
a device for sensing lamp parameters and/or measurement light parameters;
a device for specifying setpoints associated with the respective parameters;
a comparison device that, in the event that one or more of the sensed measurement light parameters deviate from corresponding setpoints, generates a signal representing the deviation and
an activation circuit receiving said signal and the activation circuit is connected to the activatable device.
It is thereby possible to determine the optimum point in time for a lamp exchange that allows a compromise between the maximum lamp lifetime and the measurement light quality necessary for a measuring instrument. The continuous sensing of lamp parameters and/or measurement light parameters, preferably of those parameters that are also read out in the optical instrument, can be performed during a measurement operation itself, so that if necessary a lamp exchange can be authorized immediately, thus guaranteeing high availability of a measurement light within the desired tolerance range.
This is critically significant specifically for production lines with a high throughput, in order to minimize production wastage. In an advantageous embodiment of the invention, the brightness or intensity of the measurement light, the frequency with which brightness or intensity fluctuations occur, and its spectral distribution, are sensed as the measurement light parameters. The method is thus suitable especially for an illumination apparatus that is used in conjunction with spectroscopic measurement methods, for example an optical layer thickness measurement.
The lamp life of lamps used in illumination devices, for example halogen lamps, xenon lamps, or deuterium lamps, is time-limited because of their design. For the aforementioned lamps, lifetimes guaranteed by the manufacturer are in the range of 1000 hours and above. In a further advantageous embodiment of the invention in this context, in order to guarantee a high degree of uniformity in the measurement light, the lamp life of each lamp is added up and the fact that a predefined lamp life has been reached is signaled, whereupon an exchange of the lamp or of a lamp group is performed.
This makes it possible, in particular, to protect against the risk of explosion, which increases toward the end of the lamp""s lifetime. Leaving this aside, it is further advantageous also to monitor the illumination apparatus for total failure of a lamp and to signal any such failure, so as thereupon immediately to initiate an exchange of the defective lamp or lamp group.
To simplify the monitoring regime, checking for total failure of a lamp or lamp group, and/or checking the lamp life, can be accomplished with a photodetector close to the lamp, so that malfunction information can be arrived at with particularly high reliability. The monitoring outlay for the aforesaid criteria moreover remains low. Also possible is a process-engineering decoupling of malfunction messages resulting from measurement light parameter deviations. It is also conceivable to monitor the lamp current so that a total failure can be identified.
In a further advantageous embodiment of the method according to the present invention, after a measurement light parameter deviation has been signaled, a check measurement is performed so that impairments of the measurement light that are not caused by the lamps can be identified and if applicable eliminated. This avoids uneconomical premature lamp exchanging. For the check measurement, first a calibration is performed on the optical measuring instrument using the optical measurement assemblies that are present in any case. An exchange of the lamp or lamp group is performed only if a deviation from the predefined parameter ranges continues to be signaled even after calibration.
The calibration is preferably accomplished on the basis of the comparison of a known spectrum of a reference body that is stored, for example, in a data processing apparatus, to a measurement light spectrum influenced by the reference body. This procedure is suitable in particular for a layer thickness measurement instrument, for example a spectrophotometer or spectroellipsometer, in which the aforementioned calibration can be performed with little effort, optionally even automatically.
In a further advantageous embodiment, alternatively or in addition to the aforementioned calibration operation a further check measurement is performed in which the optical measuring instrument is calibrated with a reference body of known layer thickness, by the fact that the layer thickness value derived from the influence on the measurement light is compared to the known layer thickness of the reference body. Only if the deviation in measurement light parameters from the predefined parameter ranges continues to exist is an exchange of the lamp or lamp groups then initiated. Otherwise the lamps or lamp groups presently in operation can continue to be used, so that the aforesaid procedure prevents any unnecessary early exchange of the lamps but also guarantees a high level of uniformity in the measurement light at the measurement point, and consequently excellent measurement accuracy in the optical measuring instrument.
In order to limit process complexity and arrive at a particular simple procedure for performing the measurement light monitoring, the sensing of lamp parameters and/or measurement light parameters is accomplished simultaneously or alternatingly with the performance of the measurement task for which the optical measuring instrument is configured, at least one of the assemblies that serves to perform the measurement task also being used to sense or monitor the lamp parameters and/or measurement light parameters.
In a further advantageous embodiment of the method, exchanging of the lamp or lamp groups is accomplished automatically. The illumination apparatus is thus suitable in particular for use in continuously operated measuring instruments that are utilized, for example, in a series production line. The lamp exchange necessary in order to maintain a high measurement light quality can then be performed, if applicable, completely without the intervention of operating personnel. thus resulting in no, or in any case minimal, delays in the production sequence. The time needed to exchange the lamps can thereby also be minimized.
The object upon which the invention is based is furthermore achieved with an illumination apparatus for an optical measuring instrument, in particular for a layer thickness measuring instrument, comprising multiple lamps serving as a measurement light source, of which at least one is provided for performing the next measurement task while the others serve as reserve lamps; an operating voltage source that can be switched on and off and is connected via contacts to the at least one lamp serving as the measurement light source; an activatable device for selectably conveying the lamps to the contacts; devices for continuous and/or intermittent sensing of lamp parameters and/or measurement light parameters; devices for specifying setpoints associated with the respective parameters; and a comparison device that, in the event that one or more of the sensed measurement light parameters deviates from the corresponding setpoints, generates a signal representing the deviation and forwards that signal to an activation circuit that is connected to the conveying device.
The advantages attained are those already described in conjunction with the method according to the present invention.
In an advantageous embodiment of this illumination apparatus, the conveying device is configured as a rotatable drum on whose circumference the lamps are arranged at radially symmetrical intervals; the contacts are in radial engagement with at least one of these lamps; and the drum is coupled to a drive that, as a function of a positioning signal, causes the drum to rotate until the lamp in engagement with the contacts has been exchanged.
This manner of achieving the object makes possible a particularly compact design for a lamp changer, on which a large number of lamps or lamp groups can be provided so that at the end of the operating life of a lamp or lamp group, the drum simply needs to be switched from one position into the next with no need to insert or remove lamps. Only when all the lamps have been exhausted is it necessary to repopulate the drum with lamps.
The radially external arrangement of the electrical contacts of the individual lamps or lamp groups moreover makes possible a considerable simplification in power delivery, which can be accomplished via a single connector apparatus.
To simplify lamp exchange by way of a rotation of the drum, the electrical connector device is configured to be movable radially back and forth with respect to the rotation axis, so that damage to the electrical contacts, especially on the electrical connector device, during an exchange operation is reliably prevented.